This application corresponds to German Application No. 100 60 679.2 filed Dec. 6, 2000, and herein incorporated by reference in its entirety.
1. Field of the Invention
This invention relates generally to a hydrostatic traction drive with a hydrostatic motor and at least one drive unit actively connected to it.
2. Technical Considerations
A traction drive is described in DE 42 06 085 A1. In that case, an axial piston motor with a constant intake volume that does not have its own motor shaft is connected on the output side with a planetary gear train that is oriented coaxially with the axial piston motor and drives the planetary gear train. The output shaft of the engaging and disengaging planetary gear train runs through the center of the axial piston motor.
It is an object of the present invention to provide a compact and improved hydrostatic traction drive of the general type described above but that has a simple construction, can be used over a wide range of speeds, and is suitable for a wide variety of potential applications.
In a hydrostatic traction drive of the invention, the motor is connected with a second motor into one modular unit and the motors are arranged coaxially to each other. At least one of the motors has a variable displacement. Between the motors and/or between at least one motor and the drive unit to which it is actively connected, there is a clutch that can be engaged and disengaged during operation.
The invention makes possible on one hand a continuously variable transmission ratio on the motor side of the hydrostatic traction drive without interrupting the traction force. The engaging and disengaging mechanical gear trains previously used to expand the speed range can be eliminated. In the low-speed range of a vehicle equipped with the hydrostatic traction drive of the invention, both motors can be initially set for the maximum displacement (intake volume). Changes in speed can be initially made on the primary side. In other words, to increase the speed, the delivery volume of one of the variable pumps feeding the motors can be increased. In this setting, the traction drive produces its maximum drive torque. To further increase the speed, the displacement of the variable motor can be reduced as the quantity of hydraulic fluid fed to the motor remains constant. The hydraulic fluid thereby not used is then available to the other motor so that the output speed of the other motor, and thus the speed of the vehicle, can be increased in the desired manner.
On the other hand, there are additional advantages as a result of the presence of engaging and disengaging clutches in the drive train. A clutch interposed between the motors makes it possible to turn off one of the motors, if necessary, and thereby reduce the power loss and wear. The drive unit can be disengaged from the motors by an output-side clutch. The types of clutches that can be used include both positive clutches, such as dog clutches, as well as non-positive clutches, such as friction clutches or viscous clutches.
In one advantageous configuration of the invention, both motors are flange mounted onto a drive unit that is realized in the form of a drive shaft. A traction drive of this type can be used, for example, as a two-wheel drive.
If there is a power take-off on both sides for the two motors that are connected with each other, an additional drive unit can be connected in addition to the drive unit mentioned above. In one aspect of the invention, this additional drive unit can be a second drive axle, for example. In that case, the motors can be located between two drive axles, and each motor can be coupled with one of the drive axles (all-wheel drive). In that case, with a clutch connected between the motors, it is possible by completely opening the clutch or by initiating a controlled slip (partial opening) to eliminate or reduce strains in the drive train between the two drive axles that can result under certain driving conditions (e.g., steering angle, different tire diameters, etc.). A controlled slip can also be used to divide the drive torques in a desired ratio between the two drive axles. An anti-slip control can also be realized, in which, by means of the clutch, the drive torque for one of the drive axles can be reduced in a controlled manner to prevent spinning or slipping of the drive wheels.
The two motors can also be integrated into one drive axle. In that case, the drive wheels of the drive axle represent drive units which are coupled with the motors, with the interposition of a step-down gearing if necessary or desired. The clutch that is located between the motors can then be used in the sense of a differential lock.
The above-mentioned effects can likewise be achieved if, as an alternative to the positioning of the clutch between the motors, there is a clutch associated with each of the two power take-offs. In addition, both motors can optionally be coupled with both or with only one of the two drive units, as a result of which when the drive axles are used as drive units instead of an all-wheel drive, if desired, a front-wheel drive or rear-wheel drive or even freewheeling (e.g., for towing operation) can be achieved. If the traction drive is used in a drive axle, both motors can be connected to one drive wheel. In this case, too, freewheeling is possible by disengaging the clutch for both motors.
If a clutch is also connected between the motors, one of the motors can be mechanically disengaged.
It is advantageous if an intermediate shaft, in particular a jointed shaft, is located between at least one drive axle and one motor. If both the motors are not flange-mounted on one of the drive axles, the connection to the two drive axles can be made by means of the intermediate shaft. The two motors are thereby integrated into the jointed shaft train.
The invention teaches that it is advantageous if both motors have their own motor shaft, and the two motor shafts are synchronously coupled to each other by a clutch sleeve that is located axially between the motors, or can be coupled to each other by a clutch that is located axially between the motors. It thereby becomes possible to use identical and essentially standardized motors.
In one advantageous configuration of the invention, the motors are realized in the form of hydrostatic axial piston motors that employ a swashplate design and are arranged symmetrically (i.e., xe2x80x9cBack to backxe2x80x9d) with respect to each other.
The construction of the hydrostatic traction drive of the invention can be simplified, and the dimensions minimized, if the two axial piston motors have a common control base receptacle and also have a common hydraulic fluid feed and discharge.
The transmission range of the hydrostatic traction drive of the invention can be increased in one development of the invention in which the displacement of both axial piston motors is variable. It thereby becomes possible to achieve an increased final speed.
In an additional configuration of the invention, at least one of the motors is reversible, which makes a further speed increase possible. In reverse, while the direction of rotation of the first motor remains the same, the direction of flow of the hydraulic fluid can be reversed in the second, reversed motor. Additional hydraulic fluid can thereby be fed to the first motor, i.e., the variable second engine, in reverse, acts as a pump driven by the first motor, the xe2x80x9cdelivery volumexe2x80x9d of which further increases the output speed. Consequently, in this operating state, the transmission range, i.e., the speed range of the hydrostatic traction drive of the invention, can be expanded.
The invention teaches that it is particularly advantageous to use the traction drive in a machine, in particular in construction equipment. On one hand, the operation of such machines at low speed, i.e., on and off the road, requires a high drive torque, and on the other hand, machines of this type must be moved as quickly as possible to the next work site, which requires them to travel at the highest possible speed on the road. The traction drive of the invention can also be used in other types of equipment, such as agricultural equipment, forest management equipment, and municipal vehicles that have one or more drive axles, just to name a few.